The team suspects the dinosaur—a type of sauropod, or plant-eating, four-legged lumberer—used its massive legs to either maneuver over hilly ground or deliver "good, hard" kicks to predators, said Wedel, assistant professor of anatomy at Western University of Health Sciences in Pomona, California.

Brontomerus—"thunder thighs" in Greek—may have even attacked like a modern-day chicken, relentlessly kicking and stomping pursuers to death, he added.

"I could only imagine how ill-tempered these sauropods would have been," Wedel said—as are most birds, dinosaurs' modern-day descendants.

In both cases, "you've got a little brain, you're permanently paranoid about all these meat-eaters around, and you're trying to protect your young."

Thunder thighs' bones were first found in 1994, when scientists rescued two partial skeletons of the then unidentified dinosaur from a fossil quarry that had otherwise been looted in eastern Utah.

When Wedel and colleagues examined the bones in 2007, they realized they'd found a new species—and an "extreme" one at that, Wedel said. For instance, the shapes of the newfound species' bones showed it had the largest leg muscles of any sauropod yet found.

The prehistoric animals roamed a landscape that would have resembled Africa's Serengeti, laced with rivers and mudholes and distinguished by vast, dry upland areas, Wedel noted. Herds of cowlike plant-eaters called Tenontosaurus would have dotted the plains. (Learn more about prehistoric animals.)

"If I could shoot you back in a time machine, it would have been like going on safari, except you'd want something more robust than a Land Rover—maybe a tank," he said.

"The sauropods were probably beautiful animals if you were a long way away with binoculars," he added.